The invention concerns a process for the control of the exposure in a photographic color copying apparatus. More specifically, the present invention relates to a process and apparatus for the control of exposure in a photographic color copying apparatus, wherein a copy original is scanned in a plurality of scanning zones photoelectrically, a measured triplet value determined for each scanning zone from the scan values obtained, corresponding to the three color transmissions or color densities of the copy original, and from the triplet value, the necessary quantities of copying light determined with consideration of certain given criteria.
Since the introduction of the first photoprinter equipped with a color scanner enormous development has taken place in the direction of more highly integrated and less expensive structural elements. Above all, the development in the field of CCD sensors (CCD=Charge-Coupled Devices) has lead to sensors with a local resolution far above that which can be processed with a justifiable investment in computer time and computer capacity, and which are equal in price or even less costly than sensors with a resolution adapted to the task at hand (for example diode arrays). It is therefore conceivable for reasons of cost alone to use a high resolution (for example CCD) sensor as a scanner, the effective resolution of which is reduced to a measure by the summation (averaging) of the measured values of locally adjacent (relative to the measured original) measuring points, that would permit the processing of the data at a justifiable expense. In the production of color copies there are "classical" problems in which even the most recent processes of density and color correction based on a relatively high scanner resolution (approximately 1,000 measuring points per negative) fail entirely or partially.
Thus, for example, exposures with bright sky portions are often copied too dark. In U.S. Pat. No. 4,279,505 a process is described whereby image areas may be recognized with a certain degree of probability that they may be sky areas, in order to partially eliminate the problems; however, this process is capable of improvement. Another problem consists of distinguishing between flash exposures and a class of counter light exposures (street canyons, doorways, etc.). In both cases, the negative has a more or less central area of high density. Both classes of exposures therefore in the known process of exposure control frequently undergo unidirectional density corrections, even though they should be treated in the exactly opposite manner ("plus correction," i.e. higher density in the case of flash exposures, "minus corrections" in the case of the aforementioned counter light exposures). An improved recognition of sky areas would have a positive effect relative to colors also, for example in beach exposures, which often are copied with too much yellow, if measuring points of these areas flow into the correction.
Fundamentally the same problems as with sky exposures are encountered in snow images. Here again, improved density corrections would be desirable.
Amateur exposures take place frequently under less than ideal illumination conditions. This is particularly true for internal exposures under incandescent light. Most processes for exposure control comprise special measures for the correction of artificial light (see for example the operating instructions for the GRETAG high capacity printers 3139 . . . 3141), without which the aforementioned class of images would be copied with a pronounced yellow color cast. However, these measures often have a negative effect on images not created under incandescent light but hardly distinguishable relative to color from them. A typical example of cases leading to such miscorrections are outside exposures of leaves in the fall, corn fields, etc.
The second important class of artificial light exposures are the so-called neon light exposures. These again are predominantly inside exposures which in most cases are copied in view of the less than ideal illumination with a pronounced greenish color cast. The correction of artificial light in this case raises the same problems as that of incandescent light. The difficultly here in that purely by color criteria it is not possible to differentiate between neon and outside exposures with green vegetation with the desired accuracy.
The problem to be solved relative to color and density correction with photographic printers may be interpreted generally as follows: initially, the parts of importance for the image of an original must be identified and subsequently the quantities of copy light must be dimensioned so that the important parts are reproduced correctly on the copy material in relation to color and density.
Thus, for example, a process is described in U.S. Pat. No. 4,668,082, and DE 35 43 076 whereby the parts important for the picture are identified by an operator, but the determination itself of the illumination parameters is carried out by a computer on the basis of the measured transmission values of the original, with consideration of the information provided by the operator. Obviously, such a process cannot be used with high capacity printers.
The present invention is intended to eliminate these difficulties and create conditions whereby the aforedescribed classic problems may be handled better.